Rivet popper hypothesis explains that

When a key species is extinct, it will lead to the extinction of several others. This species is referred to as keystone species. Paul Ehrlich tried to explain the importance of biodiversity by giving an analogy of aeroplane for the ecosystem in his ‘River Popper Hypothesis’. 

In an aeroplane (ecosystem) all parts are joined together using thousands of rivets (species). If every passenger travelling in it starts popping a rivet to take home (causing a species to become extinct), it may not affect flight safety (proper functioning of the ecosystem) initially, but as more and more rivets are removed, the plane becomes dangerously weak over a period of time. Furthermore, which rivet is removed may also be critical. Loss of rivets on the wings (key species that drive major ecosystem functions) is obviously a more serious threat to flight safety than the loss of a few rivets on the seats or windows inside the plane.